BWR instrument tube vibrations: theory of in-core detector spectra and interpretation of measurments

During 1986, in-core neutron noise measurements were made in a Swedish BWR, which indicated that instrument tube vibrations occurred in several cases. The experience gained during the interpretation of the measurements led to the setting up of a simple phenomenological model of in-core vibrating BWR detector spectra which is elaborated in the present paper. The novelty of the model is the inclusion of a global resonance-like term and its first harmonic into the noise components, representing the noise due to the detector itself vibrating in a linear and curved flux, respectively. Numerical calculations and comparison with measurements show a good qualitative agreement, which partly justifies the model itself, partly serves as a means of interpreting the measured spectra. (author)